There's currently a problem with the board displaying an error message after posting your message. Although you are not taken to the post you've just made, it generally seems to be posted anyway (although not always). So to avoid double posts, check the topic you posted in before you post again.
We're working on a quick fix, and hopefully sometime soon we'll be upgrading the board to avoid any such thing to happen again.
By the way, if you just post the word 'it' the error doesn't occur. I've tried.